The present invention relates to improvements in viewfinder devices for use in cameras and it relates more particularly to an improved viewfinder which is alternatively transferable between a first condition for low magnification and a second condition for high magnification.
In a camera constructed to change the focal length of the photographic optical system by bringing selected lenses built in a camera body in or out of the photographic optical path, i.e., a camera in which a plurality of photographic lenses of different focal lengths are selectively or alternatively brought in the photographic optical path, the viewing angle of magnification of the viewfinder device must be changed in response to changes in focal length.
Mechanism for changing the viewing angle of magnification of the viewfinder are known in the prior art, and a typical type is such that the concave lenses in an inverted Galilean finder are axially moved along the viewfinder optical axis in response to the transfer or movement of one of the lenses in the photographic optical path (hereinafter referred to as the focal length changeover operation). Another type of mechanism is constructed such that in response to the focal length changeover operation two concave lenses are alternatively inserted in the viewfinder optical path.
The former type of variable viewfinder is described in U.S. Pat. No. 3,890,626, issued on June 17, 1975 and includes a mechanism which changes the magnification of the viewfinder by the movement of the concave lens along the viewfinder optical axis, thereby continuously varying the magnification during the focal length changeover operation. This influences a photographer who is not accustomed to the camera to wrongly think that a variable focus lens or so called zoom lens is mounted in the camera since the magnification of the viewfinder gradually changes, leading to photography initiation before the lenses are completedly inserted in or retracted from the photographic optical path, the mechanism thus possessing an important disadvantage.
A variable viewfinder of the latter type is described in U.S. Pat. No. 4,038,673 issued on July 26, 1977 and includes a mechanism in which the magnification of the viewfinder is transferred alternatively to a first low magnification condition or to a second high magnification condition in response to the focal length changeover operation, the mechanism thus eliminating the disadvantages of the former type viewfinder. However, in the mechanism described in U.S. Pat. No. 4,038,673 the number of lenses constituting the viewfinder optical system is the same in the first and second conditions, thus necessitating that the surface curvatures of the concave lens inserted in the viewfinder optical path upon the viewfinder being set in the first condition for low magnification be relatively large. As a result, aberrations in the first condition of low magnification are high, the mechanism thus having the drawback that the view through the finder is of poor quality and low resolution.